Piston accumulators are in the broadest sense hydraulic accumulators used among other things to hold certain volumes of a pressurized liquid (hydraulic medium) of a hydraulic system and to return it if necessary to the system. Since the hydraulic medium is under pressure, hydraulic accumulators are treated like pressure vessels and must be designed for the maximum operating overpressure with consideration of the acceptance standards of diverse delivery countries. In most hydraulic systems at present hydropneumatic (gas-pressurized) accumulators with separating elements are used. For piston accumulators, the separating element is a piston within the piston accumulator housing, and separates a liquid chamber as the working chamber from the gas chamber as another working chamber. The working gas is generally nitrogen. The gas-tight piston largely permits decoupling from the gas chamber to the liquid chamber.
The liquid part is connected to the hydraulic circuit of the system so that when the pressure rises, the piston accumulator holds the hydraulic medium and the gas is compressed. When the pressure drops, the compressed gas expands and displaces the stored pressurized liquid back into the hydraulic circuit of the system. One advantage of a piston accumulator is that it can “work” in any position. A vertical configuration with the gas side up is preferred so that settling of dirt particles from the liquid on the seals of the piston part is avoided.
The important components of a piston accumulator are accordingly an external cylinder pipe as the accumulator housing, the piston as the separating element with its sealing system and the end-side sealing covers as cover parts containing a liquid port and a gas port, respectively. Generally the accumulator housing has two functions, first, storing the internal pressure, and second, guiding the piston within the accumulator housing. The cover parts close off the interior of the accumulator housing relative to the exterior on the end side are provided with an external thread on the outer peripheral side which can be screwed into a corresponding internal thread along the free longitudinal edge of the accumulator housing over a definable distance. Producing this threaded connection is time-consuming, making the production costs for a piston accumulator accordingly higher. Furthermore, safety measures must be taken to lock the added cover part in its position in the accumulator housing.
DE 103 03 988 A1 (corresponding to U.S. Patent Application Publication No. US 2006/0016074 A1) discloses avoiding the otherwise conventional threaded connections, and ensuring a reliable and secure connection of the cover part in the housing of the piston accumulator. For this purpose, in the disclosed solution on one side of the cover part it is fixed over the free longitudinal edge of the accumulator housing, which free longitudinal edge for this purpose undergoes a feed motion onto the cover part during the production process of the hydraulic accumulator. While avoiding the otherwise conventional screw connection solution for the respective cover part, a type of clamp seat on the respective free end of the accumulator housing is achieved. The cover part is clamped fast at least over the free longitudinal edge of the accumulator housing after its feed motion during production onto the cover part. In this connection, it is sufficient if part of the free longitudinal edge implements this clamping seat.
Although for the above-described hydraulic accumulator solution the cover part is provided with sealing means, especially in the form of gaskets, it cannot be precluded that especially at high pressures in the working chambers and/or for correspondingly long service lives the medium stored in the working chamber unintentionally travels to the exterior. Especially when using a working gas for the working chamber of the accumulator, it must be expected that portions of gas will travel to the exterior via the sealing means of the cover part. Viewed over the long-term, this gas escape degrades the operating reliability of the hydraulic accumulator. The degrading of the operating reliability occurs especially when the hydraulic accumulator with its sealing means is exposed to major temperature fluctuations of the magnitude of −40° C. to 130° C. This range of values causes the elastomer material of the sealing means generally to yield.
In the known hydraulic accumulator solutions, generally the possibility exists of adding working gas to the pertinent working chamber of the accumulator. This adding of has, however, is accompanied by the corresponding maintenance cost which is especially undesirable if the designed hydraulic accumulators are to be used in the form of a disposable solution on site within the hydraulic system only for a predetermined time. For the correspondingly designed hydraulic accumulator and depending on its application, it can be more economical to replace it terms of a disposable solution by a new one rather than maintain it on site.